Process for the manufacture of nucleosides



United States Patent US. Cl. 260-2115 1 Claim ABSTRACT OF THE DISCLOSURE A process of manufacture of nucleosides by reacting a sugar or a sugar derivative having a free carbonyl func tion with a heterocyclic base containing at least one nucleophilic nitrogen atom in the ring in the presence of polyphosphoric acid phenyl ester.

It is known that nucleosides can be obtained by heating sugar with a nitrogen base in the presence of polyphosphoric acid ethyl ester (see German patent specification No. 1,206,907). However this process has the disadvantage that it is ditficult to reproduce and does not always give satisfactory results. Moreover, when the nitrogen base contains free amino-groups, an undesired alkylation can take place.

The present invention is based on the observation that nucleosides are obtained in a simple manner and in good yield by reacting a sugar or a sugar derivative containing a free carbonyl function with a heterocyclic nitrogen base containing at least one nucleophilic nitrogen atom in the ring in an inert solvent and in the presence of polyphosphoric acid phenyl ester, of which the phenyl residue may contain lower alkyl groups or halogen atoms.

As sugars for use in the process of the invention, aldoses, such as trioses, tetroses, pentoses, hexoses and heptoses, and also the corresponding ketoses, are suitable. There may be mentioned, for example, arbinose, ribose, xylose, lyxose, fructose, glucose, allose, altrose, gulose, talose, mannose, idose, psicose, sorbose, galactose, mannoheptose, galaheptulose, erythrose and threose.

Apart from these simple sugars, derivatives containing a free carbonyl group can be employed. These include: desoxy-sugars, for example, rhamnose, digitalose, fucose and .desoxy-ribose; acylated amino-sugars, for example, N-acetyl-glucose-amine, N-acetyl-glactosamine and N- acetyl-mannosarnine; oligosaccharides, such as lactose, gentiobiose, melibose and maltose; and monocarboxylic acids of sugars, for example glucuronic acid and galacturonic acid, in the form of esters with alcohols such as methanol, ethanol, propanol, butanol and benzyl alcohol. Those sugar derivatives in which one or more hydroxyl groups are etherified, especially by methyl groups, or are esterified', for example, with acetic acid, benzoic acid or para-nitrobenzoic acid, can also be used as starting materials. In particular it must be mentioned that those apurinic acid formed by splitting off the purine residues from nucleic acids in a weakly acid solution and liberating the aldehyde function of the ribose or desoxyribose without impairing the degree of polymerisation of the nucleic acids [cf. J. Biol. Chem. 195, 49 1952)] may also be used as sugar components.

As the second reaction component there comes into consideration N-containing heterocycles having at least one nucleophilic nitrogen atom (an N-atom capable of being substituted by nucleophilic reagents) in the ring. As examples there may be mentioned derivatives of ice pyrimidine, such as uracil, dihydro-uracil and 2,4-di ethoxypyrimidine, cytosine, thymine, orotic acid, and also barbituric acid and other cyclic ureides and those derived from imidazole, such as purine, 2-aminopurine, 6-methylaminopurine, 6-dimethylaminopurine, Z-methyl-adenine, 2,6-diaminopurine, hypoxanthine, isoguanine, 2,8-dihydroxy-adenine, mercaptopurine, 6-mercapto 2 amino purine, theophylline, Z-aZa-adenine, 8-aza-adenine, 8-aza- 6-mercaptopurine, 8-aza-guanine, 'S-aZa-isoguanine, S-azahypoxanthine, 8-aZa-2,6-diaminopurine, desaza-adenine, and guanine. Derivatives of pyrazole or of triazole, and also derivatives of pyrazine, for example, the piperidines, may also be mentioned.

As condensing agents, polyphosphoric acid phenyl esters (PPP) are especially suitable. Those esters that are derived from PPP by replacing one or more of the hydrogen atoms in the phenyl residues by lower alkyl groups or halogen atoms may also be used. There may be mentioned, for example, tri-para-chlorophenyl polyphosphate and tri-para-tolyl polyphosphate.

The polyphosphoric acid phenyl esters used as condensing agents are obtained by heating phosphorus pentoxide with triphenyl phosphate or a tri-alkylphenyl or tri-halogenphenyl phosphate at a temperature above about 200 C. The duration of the reaction depends on the temperature. Thus, at 200 C. the reaction terminates after about 15 to 40 hours and at 300 C. it terminates after 2 to 5 hours. It is not essential to use the components in molar ratio. Advantageously, triphenyl phosphate is used in an excess of up to 2.5 to 1. The reaction products are at room temperature, depending on the relative proportions, clear, vitreous to highly viscous masses, which can, however, be easily handled at 50 C. They dissolve very well in chloroform, dimethyl-formamide, dimethyl sulphoxide and dioxane, but are sparingly soluble in ether, petroleum ether, benzene and water.

The probable course of the reaction is illustrated below by way of example in the preparation of adenosine.

IITH HO O CHzOH The syntheses of the nucleosides can be carried out in two ways. Either the sugar is first phosphorylated and reacts subsequently with the base, or the base is first phosphorylated and then reacts with the sugar. The latter method is preferred. The process of the invention is advantageously carried out by reacting the nitrogen base in an inert solvent with the polyphosphoric acid ester and a few drops of acid, then adding the solution of the sugar, evaporating the solvent, and heating the homo geneous reaction mixture that remains behind. As the yields are higher the larger the quantity of base used. About 1.5 to 20 parts of the base will generally be used for each 1 part of the sugar. It is indeed possible to carry out the reactions with molar proportions of sugar and base, but then some of the sugar used is lost due to side reactions. The excess of the base can be increased beyond the proportion mentioned above, for example, up

CHzOH to 100 times, without resulting in any substantial improvement in the yield. The temperatures and hydrogen ion concentrations to be chosen naturally depend on the properties and reactivities of the reactants. It is preferable to work at temperatures within the range of about 50 C. to 100 C. and at a pH-value (measured in dimethyl-formamide) ranging from 0.5 to 3.

As solvents for use in the process, those in which the sugar and the base dissolve well, and which do not themselves react with the reactants, are suitable. As solvents there may be mentioned, more especially, esters and alkylamides of various acids, for example, diethyl phosphite, phosphoric acid tris-dimethylamide, dimethyl sulphoxide, formamide or, more particularly dimethyl-forrnamide. The presence of small amounts of water is not harmful.

The products of the process are advantageously isolated by dissolving the reaction mixture in water and neutralising the solution. Unconsumed polyphosphoric acid ester remains behind undissolved and can be removed by filtration. The nucleoside can be recovered from the aqueous solution by column chromatography on Dowex formate and elution with ammonium formate solution or by simple concentration.

The use of PPP or the aforesaid substituted PPP, instead of polyphosphoric acid ethyl ester has the advantage that no alkylation of the reaction product is to be feared. Furthermore, the PPPs can be much more easily prepared and are considerably more stable, so that the manufacture of the nucleoside is simplified, the yields are improved and the formation of decomposition products is suppressed.

Owing to their pharmaco-dynamic action, the products of the process can be used as medicaments or as intermediate products for the manufacture of pharmaceutical products. Thus, for example, adenosine serves as a circulatory agent, psicofuranine and also arabinosyladenine and arabinosylcytosine (S.S. Cohen, Progr. Nucleic Acid Research 5, 1 (1966)) as cytostatics, and 2-(2'-desoxy-D- ribofuranosyl) 6 methyl-asym-triazine 3,5 (2,4) dione (Azathymidine, I. Am. Chem. Soc. vol. 80 (1958), p. 1138) acts as an inhibitor of the DNA-synthesis. The nucleic acids obtainable by the use of apurinic acids in the process of the invention exhibit a considerable influence on the cell metabolism and can be used as cytostatics, for example, as inhibitors of bacteria, or for the transformation of organisms, for exam le, for the production of virus mutants.

The following examples illustrate the invention, but they are not intended to limit it thereto:

EXAMPLE 1 Preparation of polyphosphoric acid phenyl ester 44.93 grams of phosphorus pentoxide (0.158 mol) were introduced into 131.5 grams (0.404 mol) of molten triphenyl phosphate (melting point 49 C.), and stirred well at room temperature until the initial suspension had become a stiff mass and the evolution of heat had subsided. The flask was then heated in a domed heating chamber at 300 C. After about 2.5 hours all the phosphorus pentoxide had dissolved and a homogeneous product was obtained. The latter was then stirred for a further minutes at 300 C.

EXAMPLE 2 aand {3-9-D-ribofuranosyl-adenine 5 grams of adenine (37 mmol) were dissolved in 500 ml. freshly distilled dimethyl-formamide to which 3 ml. of concentrated hydrochloric acid had been added. The pH-value of the solution as measured with a glass electrode was less than 1. After the addition of 16 grams of polyphosphoric acid phenyl ester the mixture was heated at 50 C. for 5 minutes. Then 1 gram of ribose (7.4 mmol) in 250 ml. of dimethyl-formamide were added and the solvent was distilled off in a rotary evaporator under reduced pressure at 100 C. A yellow rubber-like residue remained behind. The residue was heated for 2 to 3 minutes at 98 C. and, after being cooled, was dissolved in 25 ml. of water. the pH-value being adjusted to 7 by means of a 2 N-solution of sodium hydroxide. The solution was inserted in a cooling cabinet, and after a short time a precipitate consisting of adenine and phenyl polyphosphate separated out and was filtered otf. The filtrate was concentrated, and adjusted to a pH-value of 11 by means of an ammonia solution of 25 percent strength. In order to isolate the nucleoside fraction, a column x 3.6 cm.) filled with Dowex-lXlO-formate which had been equilibrated with a 0.01 molar solution of ammonium formate was used. The filtrate was fed to the column and elution was carried out at a pH-value of 10.2 with a 0.01 molar solution of ammonium formate. The eluate was collected in fractions of 20 ml. and in each fraction the ultra-violet absorption was determined. The fractions 41 to 65 contained principally oc-EldCIlOSlI'lC and fractions 66 to 103 contained principally fl-adenosine.

For further purification the two main fractions were again chromatographed on a column of Dowex-lXZ-OH (50 x 3.6 cm.) with the use of methanol of strength as elution agent. The fractions 66 to 153 from the Dowex formate column, which contained mainly fi-adenosine, were concentrated by evaporation and chromatographed in the manner described above. Fractions 78 to 100 contained the residual a-adenosine, and the fi-adenosine appeared in fractions 165 to 220. The latter fractions were concentrated by evaporation, and the dry residue was dissolved in methanol. The fl-adenosine crystallised out from the concentrated solution. After repeated recrystallisation from methanol the compound melted at 234 to 235 C. It had the specific rotation [m] 61.6. These data agree with the values given in the literature. In order to purify the wadenosine, fractions 41 to from the Dowex formate column were chromatographed on Dowex-1X2-OH, and the eluate was collected in 20 ml. fractions. The course of the chromatography was followed by paper chromatography and ultra-violet spectroscopy. Pure 1xadenosine appeared in the vicinity of fraction 80, and pure fl-adenosine followed later. The fractions containing a-adenosine were collected and concentrated by evaporation. The residue was dissolved in methanol and the solution was concentrated to a volume of 1 to 2 ml. By the addition of about 10 ml. of ethyl ether the a-adenosine was precipitated in a crystalline form. After recrystallisation from a mixture of methanol and ethyl ether, it melted at 201 to 202 C. and had the specific rotation [a] 29.5 (in water).

aand B-adenosine were obtained in approximately equal quantities. The total yield of the two anomers amounted to 18 to 20% calculated on ribose.

EXAMPLE 3 aand 3-9-D-desoxyribofuranosyl-adenine 500 m. (3.7 mmols) of adenine were dissolved in ml. of dimethyl-formamide to which 0.3 ml. of concentrated hydrochloric acid had been added. After the addition of 1.3 grams of polyphosphoric acid phenyl ester the mixture was heated for five minutes at 50 C. Then 0.1 gram of 2-desoxyribose (0.67 mmol) in 50 ml. of dimethyl-formamide was added, and the pH-value was adjusted to 2 to 2.5 by the addition of about 0.33 ml. of triethylamine. The solvent was distilled off under reduced pressure in a rotary evaporator at a bath temperature of 50 C., and the slightly yellowish residue was heated for a further 10 minutes at 50 C. It was then dissolved in 5 ml. of water, and the solution was neutralised with a 2 N-solution of sodium hydroxide. During cooling in a cooling cabinet, unconsumed phenyl polyphosphate precipitated out in a short time, and the precipitate was removed by filtration. The filtrate was adjusted to a pH- value of 10.5 by means of a solution of ammonia of 25 percent strength. The solution was chromatographed at a pH-value of 9.6 on a column of Dowex-1X10 formate (28 x 2.6 cm.), and the eluate was collected in 15 ml. fractions. Fractions 11 to 60 contained a mixture of aand B-desoxyadenosine as determined by paper chromatography in a mixture of butanol and water.

For further purification, the mixture of the anomers was concentrated to a volume of 3 to 5 ml. and chromatographed over a column of Dowex-lXZ-OH with the use of methanol of 30 percent strength as elution medium. The eluate was collected in 20 ml. fractions. Pure 0cdesoxyadenosine appeared in fractions 46 to 60. After removing the solvent, the residue was dissolved in a small quantity of methanol, precipitated with ethyl ether, and then recrystallised from methanol. The product melted at 209 to 211 C., and had a specific rotation [11] of 71 (in water).

B-desoxyadenosine appeared in fractions 77 to 90. After evaporating the solvent, the residue was recrystallised from 1.5 ml. of water with the addition of a few drops of ammonia. The ,B-desoxy-adenosine so purified melted at 188 to 189 C. and had a specific rotation [oc] of 26.

The total yield of uand fi-desoxy-adenosine amounted to 35 to 40 percent, calculated on desoxyribose. The quantity of u-desoxy-adenosine was somewhat greater than that of fi-desoxyadenosine.

We claim:

1. In a process for the manufacture of nucleosides by reaction of a sugar or sugar derivative having a free carbonyl group with an N-heterocyclic base having at least one nucleophilic nitrogen atom as the heterocyclic atom in the ring, in the presence of a polyphosphoric acid ester, the improvement wherein said polyphosphoric acid ester is a polyphosphoric acid phenyl ester or a polyphosphoric acid phenyl ester in which the phenyl groups are substituted with lower alkyl groups or halogen atoms, said esters being prepared by heating triphenyl phosphate, a tri-lower alkyl phenyl phosphate, or a tri-halophenyl phosphate, respectively, with phosphorus pentoxide at a temperature above about 200 C.

References Cited UNITED STATES PATENTS 3,278,518 10/1966 Schramm et a1 260-211.5

LEWIS GOTTS, Primary Examiner J. R. BROWN, Assistant Examiner 

